1. Field of Invention
This invention relates to MOS devices and describes effects of chromium doping on ionization damage in MOS devices and a process to achieve hardening against radiation for such devices.
2. Prior Art
A number of studies have been made concerning ionizing radiation damage in metal-oxide-semiconductor field-effect transistors (MOS FET's). Such studies are: (1) E. H. Snow, A. S. Grove, and D. J. Fitzgerald, "Effects of ionizing radiation on oxidized silicon surfaces," Proc IEEE, vol. 55, pp 1168-1185, July 1967; (2) H. L. Hughes and R. R. Giroux, "Space radiation effects MOS FET's," Electronics, vol. 37, No. 32, pp 58-60, 1964; (3) J. P. Mitchell and D. K. Wilson, "Surface effects of radiation on semiconductor devices," BSTJ, vol. 46, pp 1-80, January 1967; (4) K. H. Zaininger and A. G. Holmes-Siedle, "A survey of radiation effects in metal insulator semiconductor devices," RCA Review, pp 208-240, June 1967; and (5) E. Kooi, "Effects of ionizing irradiation on the properties of oxide-covered silicon surfaces," Philips Research Reports, vol 20, pp 595-619, 1965. These studies have shown that the ionization and trapping of charge carriers in the gate oxide are responsible for much of the radiation damage. Additional damage occurs because of an associated increase in fast surface state density. The electrical manifestations of ionization damage in MOS FET's are a change in threshold voltage (the gate voltage required for conduction) and a decrease in transconductance due to the additional surface states. Serious device damage and circuit failure can occur at relatively small ionizing radiation doses. At present, the use of MOS FET's in ionizing environments is limited by this sensitivity to ionization effects.
A number of approaches have been suggested which might improve the radiation hardness of MOS FET's. These involve either the use of a new gate dielectric material or some method of modifying the presently used silicon dioxide so as to reduce net charge trapping. These approaches have been stated in: (1) K. H. Zaininger, A. G. Homes-Siedle, and W. J. Dennehy, "Radiation effects in complementary MOS transistors," Tech. Reports under AFAL Contract F33615-67-C-1140, 1968; and (2) A. G. Stanley, "Comparison of MOS and metal-nitride-semiconductor insulated gate field effect transistors under electron irradiation," IEEE Trans. Nuclear Science, vol. NS-13, pp 248-254, December 1966. The most promising methods of modifying the silicon dioxide consist of doping the gate oxide with a multivalent metal. Titanium, chromium, and aluminum have been used for this purpose. Such methods have been described in: (1) J. Lindmayer and W. P. Noble, Jr., "Radiation resistant MOS devices," IEEE Trans. Electron Devices, vol. ED-15, pp 637-640, September 1968; (2) R. Baer and D. Long, "A new radiation hard MOS transistor," IEEE An. Conf. on Nuclear and Space Radiation Effects, Columbus, Ohio, 1967.